This invention relates to carrier wave intelligence systems in general and, more particularly, to apparatus useful in detecting and extracting information or intelligence transmitted outbound to electricity meters and the like over electric power distribution networks from a central site.
The use of electric power lines for meter reading, load control, and other communications purposes is well known in the art. It is known that a modulation voltage can be superimposed on a power system voltage to cause wave shape perturbations in the carrier wave. In the embodiment described hereinafter, the carrier wave is the voltage wave of an electrical power distribution system or network. Such systems are described in U.S. Pat. Nos. 4,106,007, 4,218,655, and 4,400,688 to Johnston et al, and 4,105,897 to Stratton et al, which patents are incorporated herein by reference.
Communication over an electric distribution network is a complex undertaking. Each customer service constitutes a branch in the distribution feeder, and the branching is so extensive that it is impractical to provide filter and by-pass circuitry at each branch point. The distribution network is not an attractive medium for conventional communications due to the attenuation and dispersion of the signals and because noise levels tend to be high. To overcome the high noise levels, it is generally necessary to use narrow band filtering, error-detecting and error-correcting codes, and relatively high signal power levels at low bit rates.
The aforementioned problems arise in two areas. The first, to which the present invention relates, concerns transmitting information from the central source in the direction of energy flow to the individual customer premises. This transmission of information in the direction of energy flow is referred to as "outbound" signaling. Information flow in the opposite direction, from customer to central site, is called "in-bound" signaling.
For "outbound" signaling, in order to reach line-to-line customers on the three-phase distribution network of a utility, for example, the modulation signal which carries the information preferably should have dominant positive and negative sequence components. This implies that the outbound modulation signal should not appear on all three phases simultaneously at equal strength and phase relationship.
At least on outbound signal detector system looks for signals disposed on the voltage carrier at the -10.degree. and the +30.degree. points on the waveform. A fixed signal threshold is typically used with that system to determine the presence or absence of signal at the detection points. This system has shown good performance under various conditions, but it could be improved.
At least one area of possible improvement concerns coping with the dynamics of the distribution network. For example, outbound signaling causes transient oscillations in the waveform which depend on the capacitance and load on the network at that time. Variation in loads results in a great variation in these transients, with resulting distortion of the waveform. Since loads on power distribution networks vary with time of day, this means that the reliability of the outbound signal detector can also vary with the time of day.
U.S. Pat. No. 4,914,418 to Mak et al., the disclosure of which is hereby incorporated herein by reference, describes one approach to coping with the dynamics of the distribution network. But under certain network conditions, even the Mak et al. approach could be improved.
A second possible area of improvement relates to crosstalk. In any three-phase system (which power distribution networks typically are), the voltage in any one phase is related to or coupled to the voltages in the other two phases. This leads to crosstalk. It should also be realized that the source configuration of the power distribution network also affects the severity of crosstalk.
A study of various source configurations reveals that the outbound signal around the voltage zero crossing changes in magnitude and frequency with respect to the zero crossing and depends on network loading. Moreover, during certain loading conditions crosstalk may be severe or less severe, creating difficulties in signal detection and identification. It has also been found that one type of crosstalk is due to the trailing end of the oscillatory signal wave.
It should be appreciated that the difficulty of detecting the outbound signal is further complicated by the fact that such detection normally takes place at a remote location (such as the electricity meter for a user) which has only limited space available. Moreover, for such detectors to be widely used they must be relatively low in cost.